The present invention relates to explorations for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. Specifically, the present invention relates to imaging the wall of the borehole with highly localized currents.
So call micro-resistivity, or micro-conductivity, techniques have been used in open, or uncased, borehole logging to obtain a two dimensional image of the borehole wall. The techniques allow for the evaluation of the characteristics of the earth formations that are penetrated by the borehole. Structural and stratographic analysis of the borehole is improved. The techniques allow identification of thin beds, fractures, and faults and provide information on porosity. This information is useful in determining if the borehole has penetrated a hydrocarbon (for example oil or gas) bearing formation and whether the hydrocarbons are commercially extractable.
Micro-resistivity techniques utilize a tool having a number of deployable pads (six pads are commonly used). As the tool is run down the borehole, the pads are retracted. When the tool is readied for logging, arms extend the pads radially out. Each pad has a number of button electrodes. The buttons are closely arranged in two horizontal rows (when the tool is vertical). Current is introduced into the borehole wall through the individual button electrodes. Electronics on board the tool measure the variation in current through the individual electrodes. These measurements are then processed and interpreted to form the image of the borehole wall.
When the pads with the sensors are stowed, each pad takes up less than 120 degrees of circumference. The pads are vertically staggered when stowed. Thus, there are three upper pads and three lower pads. The pads circumferentially overlap each other. However, when the pads are deployed radially outward, there are circumferential gaps between the pads. The size of the gaps depends on the diameter of the borehole. For large diameter boreholes, the pads will be extended further radially outward than for a small diameter borehole. This is because the pads contact the borehole wall.
By design, the spatial resolution of each button electrode is small in order to increase the image resolution. Because of the small resolution of the button electrodes and the small radial spacing of the pads, the image that is produced has gaps therein (see FIG. 4). In large diameter boreholes, the gaps represent as much as one-third of the surface area of the borehole wall. The gaps degrade the quality of the image.
The present invention eliminates the gaps in the image (see FIG. 5). The present invention uses more pads that are circumferentially offset so as to cover the gaps. Using more pads generates more data that must be sent to the surface over a limited bandwidth wireline.
The present invention uses the combination of two or more tools to provide more coverage of the borehole. In common borehole sizes, the coverage is full, or 100%. In larger boreholes, the coverage may not be full, but will be greater than with the prior art.
The present invention provides a downhole tool method for obtaining a more complete circumferential image of a borehole wall using micro-resistivity techniques.
The present invention provides a downhole tool for imaging a circumference of the wall of a borehole with micro-resistivity measurements. The tool comprises an elongated body having a longitudinal axis, a first set of pads and a second set of pads. Each of the pads of the first and second sets of pads has micro-resistivity electrodes for micro-resistivity measurements. Each of the pads of the first and second sets of pads is mounted on a respective arm. Each of the pads of the first and second sets of pads is moveable from a respective stowed position against the body of the tool to a radially extended position, wherein in the extended position, the pad is structured and arranged to contact the borehole wall. Each of the pads in the first set of pads is circumferentially separated from the respective adjacent pads in the first set of pads by a gap when the first set of pads are in the extended position. The pads in the second set of pads are circumferentially aligned with the gaps of the first set of pads.
In accordance with one aspect of the present invention, the first set of pads is longitudinally spaced from the second set of pads.
In accordance with another aspect of the present invention, the tool comprises a tool stack. The first set of pads are provided by a first tool and the second set of pads are provided by a second tool. The tool stack comprises the first and second tools.
In accordance with still another aspect of the present invention, the first tool is separated from the second tool by an isolator.
In accordance with still another aspect of the present invention, the pads of the first set of pads are longitudinally separated from the pads and arms of the second set of pads.
The present invention also provides a downhole tool stack for imaging a circumference of a wall of a borehole with micro-resistivity measurements that comprises a first tool and a second tool that is longitudinally positioned from the first tool. Each of the first and second tools have plural pads, with each pad having micro-resistivity electrodes for micro-resistivity measurements. Each pad is mounted on a respective arm. Each pad and respective arm are moveable between a stowed position and an extended position. The pads are structured and arranged to contact the wall of a borehole. The pads of the first tool are circumferentially separated from each other by gaps when the pads of the first tool are in the extended position. The pads of the second tool are circumferentially aligned with the gaps.
In accordance with another aspect of the present invention, the first tool produced first data and the second tool produces second data, with the tool stack further comprising a telemetry module that polls the first tool at a first rate so as to receive the first data for telemetry uphole and that polls the second tool at a second rate so to receive the second data for telemetry uphole. The telemetry module modifies the rate of polling from the first and second rates so as to correspond to the quantity of data received as a result of the polling.
The present invention also provides a method of assembling a tool stack for imaging a circumference of a wall of a borehole with micro-resistivity measurements. The first tool is provided with deployable micro-resistivity pads. The first tool has first and second ends. A second tool is connected with the second end of the first tool. The second tool has deployable micro-resistivity pads; the pads of the second tool are circumferentially offset from the pads of the first tool. A wireline is connected with the first end of the first tool.
The present invention also provides a method of imaging a wall of a borehole with micro-resistivity measurements. The method acquires imaging data of first portions of the borehole wall from a first tool and also acquires imaging data of second portions of the borehole wall from a second tool. The second portions are interposed between the first portions.
In accordance with one aspect of the present invention, the step of acquiring imaging data for the first portions of the borehole wall from a first tool and acquiring imaging data of second portions of the borehole wall from a second tool are performed simultaneously, with the imaging data of the second portions being laterally offset from the imaging data of the first portions.
In accordance with another aspect of the present invention, the step of acquiring the imaging data of first portions of the borehole wall from a first tool further comprises the step of polling the first tool for the first tool imaging data at a first rate. The polling rate is modified from the first rate to a second rate.
In accordance with still another aspect of the present invention, the step of acquiring imaging data of second portions of the borehole wall from a second tool further comprises the step of polling the second tool for the imaging data at a third rate. The polling rate is modified from the third rate to a fourth rate.
The present invention also provides a method of imaging a wall of a borehole. A logging tool is provided in the borehole. The logging tool has a first set of pads and a second set of pads. The pads of the first set of pads have gaps therebetween. The logging tool is moved along the borehole. At a first depth, the borehole wall is logged with the first set of pads. The logging tool continues to move along the borehole. At the first depth, the borehole wall is logged with the second set of pads, the pads of the second set of pads being located in the gaps of the first set of pads at the first depth.
The present invention also provides a method of telemetering data from a downhole tool to the surface. Data is acquired from the tool by polling the tool at a polling rate. The amount of data received from the tool is determined. The polling rate is modified to correspond to the amount of data received from the tool, wherein if the amount of data received from the tool increases, the polling rate increases.
In accordance with one aspect of the present invention, the step of acquiring data from the tool further comprises the step of receiving a packet of data from the tool. The step of determining the amount of data generated by the tool further comprises the step of determining how much data is in the packet.
The present invention also provides an apparatus for telemetering data from a downhole tool stack having plural tools to the surface. The apparatus comprises means for acquiring data from one of the tools at a polling rate, means for determining the amount of data received from the tool as a result of polling and means for modifying the polling rate to correspond to the amount of data received from the one tool, wherein if the amount of data received from the one tool increases the polling rate increases.